PROJECT SUMMARY ABSTRACT Aging progressively deprives us of many essential functions, including those of our central nervous system (CNS). Neuroinflammation is linked to aging-associated cognitive impairments and dementia-related neurodegenerative diseases, such as Alzheimer's disease. Therefore, it is essential to elucidate the mechanisms underlying increased inflammation in the aging brain, as a means to develop strategies to combat its deleterious effects on cognitive function. Microglia, the resident innate immune cells of the CNS, mediate neuroinflammation and play essential roles in the establishment and refinement of neuronal circuits. In particular, microglia incorporate signals from the microenvironment, such as neurons and systemic factors, to initiate appropriate neuron-supporting responses. On the other hand, it is also known that microglial dysfunction adversely affects neuronal and cognitive functions, including processes susceptible to the aging process, such as learning and memory. Indeed, microglia immune responses have been shown to directly contribute to cognitive dysfunction in models of age-related neurodegenerative diseases, such as Alzheimer's disease. Given the connection between aged microglia and their microenvironment, together with their proposed role in the genesis of cognitive deficits under neurodegenerative disease conditions, mechanistic insight into what drives microglia aging is necessary to discover means to counteract the vulnerability of the aged brain to functional decline. Therefore, this proposal aims to test the hypothesis that age-related changes in the adult brain microenvironment promote aging and rejuvenation of microglia. The intent of the proposed study is to investigate the influence of the aging microenvironment on microglial inflammatory phenotypes by providing a detailed molecular and functional profile of age-related microglial responses to their changing microenvironment. This will be investigated with three Specific Aims. In Aim 1, the molecular heterogeneity of microglia in the aging hippocampal region of the brain will be characterized. Additionally, the molecular changes that occur during aging will be used to determine the processes that are disrupted in microglia of the aging hippocampus. In Aim 2, the role of the aging microenvironment in directing inflammatory phenotypes in young microglia will be determined using innovative and highly technical approaches. Lastly, in Aim 3 the sufficiency of a young microenvironment to rejuvenate aged microglia will be examined using the approaches developed in Aim 2. Successful completion of the proposed research will provide an in depth understanding of the molecular and cellular dynamics of microglial aging, as well as determining the influence of the aging hippocampal microenvironment on the microglial aging process. These studies will have translational potential for targeting neuroinflammation in the aging brain and neurodegenerative diseases, such as Alzheimer's disease, by restoration of microglial function.